I. Field of the Invention
The present invention relates to digital communications. More particularly, the present invention relates to a novel and improved method and apparatus for canceling echo in a Wireless Local Loop telephone system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is but one of several techniques for facilitating communications in which a large number of system users are present. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, (the '307 patent) entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", and U.S. Pat. No. 5,103,459, (the '459 patent) entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", both of which are assigned to the assignee of the present invention and incorporated by reference herein.
In the '307 and '459 patents, a multiple access technique is disclosed wherein a large number of mobile telephone system users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations (also known as base stations, or cell-sites) using code division multiple access (CDMA) spread spectrum communication signals. The use of CDMA techniques results in a much higher spectral efficiency than can be achieved using other multiple access techniques.
Wire based or wire line telephone systems are the traditional method of providing non-mobile telephone service. Wire based telephone systems are well suited for this task because they provide high quality telephone connections at a low operating cost and can easily accommodate increased demand for telephone service by the introduction of new telephone wires into the existing network. A disadvantage of wire based telephone systems is the substantial infrastructure and capital necessary for their implementation. These requirements can make wire based telephone systems uneconomical for providing telephone service in areas that are remote or less populated, or in less developed areas where the required capital is not available. The result is that these less populated and developed areas often lack adequate telephone service. Also, the time necessary to put the required infrastructure in place makes wire based telephone systems less desirable in areas where new telephone service must be provided quickly.
Wireless cellular telephone systems require substantially less capital and infrastructure than wire based telephone systems and therefore provide a possible alternative for providing primary telephone service in areas where access to wire based telephone service is inadequate. Such wireless cellular systems are known as Wireless Local Loop (WLL).
In a WLL system, subscriber units are available in several varieties. One such subscriber unit is called a Integrated Subscriber System (ISS), which looks like an ordinary home telephone, except for an antenna which is used for transmission to and reception from the base station. The ISS unit contains a handset and a keypad, looking and functioning like an ordinary home telephone. Speech is provided from the integrated handset to the ISS base unit and converted into digital format so that it can be processed in accordance with CDMA techniques, as described in aforementioned patents '307 and '459.
Another type of subscriber unit is the Single Subscriber System (SSS) which provides for one or more inputs for standard telephone sets. An SSS unit can be used in a home where multiple standard telephone sets are used, each telephone having the same ESN so that only one call may be placed or received at one time. Users can pick up a second line and actively participate in the conversation, each home user able to communicate with the far end speaker as well as each other, similar to multiple phone extensions in the home.
Another type of WLL subscriber station can be described as a combination of ISS and the SSS, described above. The combined unit, called ISS/SSS looks like a standard home telephone, complete with a handset and keypad, but has additional RJ-11x input jacks to accommodate the addition of standard analog telephones. ISS/SSS, being a WLL telephone, is a wireless unit with respect to communications between the unit and a remote base station. ISS/SSS finds use in homes located in remote locations having no standard telephone service, where benefits of such systems outweigh the costs and delays of installing traditional land-line telephone systems.
Echo is a problem that must be addressed for both ISS and SSS WLL subscriber stations. In an ISS unit, an earseal echo is generated at the ISS handset causing the far end speaker to hear an annoying echo of his own voice. In an SSS unit, an echo results from the impedance mismatch in a 4-to-2-wire converter, known as a hybrid, within the SSS unit. The 4-to-2 wire conversion is needed so that standard analog telephones can be interfaced to the SSS.
For both ISS and SSS systems, the echo problem is alleviated using a single network echo canceller located inside the ISS or SSS unit. An example of a network echo canceller is disclosed in U.S. Pat. No. 5,307,405 (the '405 patent) entitled "NETWORK ECHO CANCELLER" assigned to the assignee of the present invention and incorporated by reference herein. In an ISS unit, the echo canceller can be optimized to cancel the earseal echo. In an SSS unit, the echo canceller can be optimized to cancel the hybrid echo.
In the ISS/SSS unit, use of a single echo canceller has several drawbacks to it, especially during conference calls where one speaker is using the ISS handset and a second speaker is using an analog telephone, where both speakers are talking to a far end user. If one ISS/SSS speaker hangs up during the conversation, the single echo canceller must adapt its filter coefficients to meet the changed echo channel characteristics. Similarly, if another speaker picks up another line at the ISS/SSS unit, the echo canceller must adapt its filter to the new echo condition.
Another problem with a single echo canceller in the ISS/SSS unit is its relatively long convergence time. The echo canceller can only update its filter coefficients when all ISS/SSS speakers are silent. The more speakers that use the ISS/SSS unit, the less time there will be silence from all parties and therefore the longer it will take for the echo canceller to adapt so as to adequately cancel the echo signal to the far end user.
What is needed is a way to cancel the echo to the far end user while eliminating the need to adapt the filters of the echo canceller when users are added or removed from the ISS/SSS unit. Other desirable characteristics would include fast convergence time and the ability to isolate each type of echo signal in order to optimize echo canceller performance.